It is desirable in many applications to be able to control as accurately as possible the temperature of a heating device whether used in industrial or laboratory processes, for example. Often such thermal control is exercised by the use of a conventional thermostat which is placed in an appropriate location with respect to the heater, so as to measure the temperature and to provide control of the application of heater voltage, in accordance therewith. Where precision control is required, the construction and operation of the thermostat may be relatively complex and costly, and the use of a single thermostat may be inappropriate where the heating volume is relatively large.
It is desirable to obtain a heater control system which not only is relatively inexpensive, but which provides precise control of the application of the heater voltage in applications where accurate heating is required.
One particular device which has been suggested is found in U.S. Pat. No. 3,524,968, issued on Aug. 18, 1970, to W. J. Walsh. In such a heating device the heating element thereof, which has a predictable positive thermal coefficient of resistivity, is connected in a bridge circuit that is part of a control circuit for energizing the heater element in varying degrees in accordance with changes in the resistivity of the heating element. The use of the heater resistance as the sensing element tends to afford more accurate thermal control than does the use of a temperature sensing thermostat. The Walsh system, however, uses an excitation device which provides a relatively high current, rapidly decaying pulse by the charging and discharging of a capacitor means. Because the sensing element used therein is relatively insensitive to small changes in heater resistance, a high current is required to obtain the desired operation thereof. Moreover, the use of a capacitance charging and discharging device makes the overall control system subject to stray capacitance or inductance which may be present in the heater lines or the controller which causes inaccuracies in the resistance measurements.
It is desirable to provide a more accurate, low power consumption device for achieving the kinds of accuracy required in sensing the heater resistance to produce such thermal control in a manner which is more advantageous and less costly than that disclosed by the Walsh patent.